As the gas turbine engine art developed, the high temperature technology advanced and more sophisticated materials and air cooled components were provided to withstand more strenuous high temperature operating conditions. One example of such area of development related to turbine components such as blades and vanes and their associated members such as shrouds. During operation, such articles experience severe temperature and environmental conditions as well as high thermal stresses, some of which result from proximity of the article body to cooling passages.
One aspect of such area was the development of articles of nickel base superalloys, particularly those manufactured to have a directionally oriented microstructure. Generally, these microstructures are referred to as having directionally elongated grains from directional solidification of the body during casting, or as having a substantially single crystal structure as a result of selecting a single grain for solidification during casting along with directional removal of heat during solidification from a melt to grow the single crystal.
It will be appreciated that use of such materials and manufacturing methods resulted in a costly article. Generally such articles include one or more coatings, or surface or edge coverings for environmental protection, erosion control, sacrificial abrasion, etc. If, during manufacture or as a result of operation in an engine, a structural discontinuity, such as a crack, or a wear or erosion area, occurred in a surface portion of the article, it was deemed more desirable and more cost effective to repair rather than replace the article.
Prior to the present invention, repair methods and materials have been reported for repair of articles made from superalloys such as those generally known in the art as nickel base superalloys. However, particularly with regard to articles having bodies of a directionally oriented microstructure operating at temperatures higher than those earlier reported components, known repair systems were inadequate. For example, they were not balanced for repair of an article manufactured to include a protective surface system of some kind. The known repair system was not controlled for "burnout capability"--control of the lowest melting point component or phase of the repair system. For example: if a brazing alloy used in manufacture was a multiple powder system, a lower melting phase would melt during subsequent repair coating or operation; if the repair system attempted to include a higher brazing temperature to withstand such higher temperature operation, the brazing temperature was so high, or the required brazing time at temperature was so long, as to result in recrystallization of the microstructure of the body being repaired, thereby reducing mechanical properties of the body. Some reported brazing systems attempted to match the composition of the repair alloy to the composition of the article. Others used melting point depressants which avoided use of such elements as Si which, in the known compositions, were seen to be detrimental to mechanical properties.